Miniaturized pressure sensor

ABSTRACT

An entirely surface micromachined free hanging strain-gauge pressure sensor is disclosed. The sensing element consists of a 80 μm long H-shaped double ended supported force transducing beam ( 16 ). The beam is located beneath and at one end attached to a square polysilicon diaphragm ( 14 ) and at the other end to the cavity edge. The sensor according to the invention enables a combination of high pressure sensitivity and miniature chip size as well as good environmental isolation. The pressure sensitivity for the sensor with a H-shaped force transducing beam, 0.4 μm thick was found to be 5 μV/V/mmHg.

[0001] The present invention relates to miniaturized pressure sensors.More specifically it relates to such a sensor wherein the sensitivityhas been substantially improved by providing a force transducing beaminside a vacuum cavity. The sensitivity is improved by the mechanicalleverage effect obtained by making the beam suspended by a diaphragm inone end point. In a preferred embodiment it relates to a pressure sensoremploying a strain gauge as the pressure transducing element

BACKGROUND OF THE INVENTION

[0002] Blood pressure measurement in the coronary artery is mostcommonly performed using a fluid filled catheter that transfers thepressure to a external pressure sensor or by a catheter based pressuresensor inside the artery. By measuring the pressure with a miniaturesensor inside the artery the time response as well as the accuracy canbe improved. The miniature sensor based measurement is well suited foruse in balloon angioplasty procedures.

[0003] Pressure sensors intended for use in medical catheter basedintravascular applications needs to be ultra miniaturized. To achieve areliable measurement in the blood vessels the pressure sensors alsoneeds to have a built in pressure reference. Vacuum or a low pressure ispreferred to minimize temperature drift. Several techniques using MEMStechnology have been proposed with optical reflective andinterferometric, capacitive and piezoresistive detection. Piezoresistivedetection technique is more favorable than capacitive detection forminiaturized sensors due to better scaling characteristic. An earlierreported absolute pressure sensor uses a piezoresistive strain gaugeprovided on top of a pressure deflectable diaphragm.

[0004] A successfully commercialized ultraminiaturized absolute pressuresensor is using the piezoresistive detection technique. The sensoraccomplishes a leverage effect by separating the strain-gauge from thediaphragm with an insulation layer to obtain an increase of thesensitivity. However, the thickness of such an insulator also stiffensthe diaphragm, thus reducing the sensitivity.

SUMMARY OF THE INVENTION

[0005] Thus, in view of the restricted sensitivity of the abovediscussed prior art sensor, the inventors have sought to develop animproved sensor. This has now been achieved by a sensor comprising avacuum cavity sealed by a diaphragm, in which a leverage of thedeflection of the diaphragm caused by the pressure exerted thereon, isachieved by suspending a force transducing beam in the diaphragm insidethe vacuum cavity. The leverage brings about an amplification of thesignal from the strain gauge on said beam.

[0006] The deflection is sensed in one embodiment by a H-shapedfree-hanging strain-gauge which is located beneath, and fully enclosedby, the diaphragm, inside the reference vacuum cavity. This enables aleverage effect without the need for an insulation layer, which wouldintroduce unwanted stiffening. The strain-gauge is thin and onlysupported at the beam ends (reffered to by the expression“free-hanging”) to further reduce stiffening effects and therebyincreasing the pressure sensitivity.

[0007] Also, the sensor is fabricated using only surface micro machiningtechniques.

[0008] The novel pressure sensor according to the invention is definedin claim 1.

[0009] Namely, the miniaturized pressure sensor, comprises a supportbody having an upper surface and a depression formed in said surface, adiaphragm, covering said depression so as to form a closed cavity, saiddiaphragm being responsive to external pressure by being deflected, astrain gauge attached at one end to said support body inside said cavityand beneath said diaphragm, and at another end suspended by saiddiaphragm.

[0010] Alternatively, the miniaturized pressure sensor, comprises asupport body with an evacuated cavity formed therein, a diaphragmforming a sealing cover for said cavity, and a strain gauge coupled tothe support body and said diaphragm such that a deflection of saidmembrane caused by a pressure exerted thereon causes a detectable changein strain in said strain gauge.

[0011] Preferably, the miniaturized pressure sensor, comprises adiaphragm responsive to pressure by being deflected, and a free-hangingstrain gauge responsive to strain by yielding an electrical outputsignal, said strain gauge being suspended at one end by said diaphragm,such that a deflection of said diaphragm causes a detectable change instrain in said strain gauge.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The invention will now be described in detail with reference tothe drawings, in which

[0013]FIG. 1 illustrates a prior art sensor;

[0014]FIG. 2 schematically illustrates the principle of the presentinvention;

[0015]FIG. 3 schematically illustrates an embodiment of the invention;

[0016]FIG. 4 is a perspective view with portions broken awayillustrating an embodiment of the present invention;

[0017]FIGS. 5-8 show steps in the process of manufacturing a sensoraccording to the invention;

[0018]FIG. 9 illustrates the improved sensitivity of the sensoraccording to the invention;

[0019]FIG. 10 illustrates a further embodiment of the inventionproviding increased sensitivity;

[0020]FIG. 11 shows a prior art design for temperature compensation;

[0021]FIG. 12 shows temperature compensation according to the invention;and

[0022]FIG. 13 is a graph showing improved relative temperaturedependency mismatch for the invention compared to prior art.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0023]FIG. 1 shows a prior art piezo resistive pressure sensor 1, wherethe strain gauge 2 comprising a piezo resistive element is provided ontop of the diaphragm 3 with an insulating layer 4 disposed therebetween.The diaphragm covers a reference vacuum cavity 5. As already indicatedabove, this insulation layer, although increasing the sensitivity tosome degree, also causes a stiffening of the diaphragm, whichcounteracts the effect of the insulation layer.

[0024] In FIG. 2 a sensor according to the invention is schematicallyillustrated in cross section. It comprises a diaphragm 6 covering areference vacuum cavity 7. Within the cavity 7 and suspended by thediaphragm 6 by means of a finite suspension member 9 at one end, a forcetransducing beam 8 is provided. The beam 8 is attached at its other endinside the cavity such that there will be a finite distance between thediaphragm and the beam. However, it is also possible to make thediaphragm 6 and beam 8 so as to be located in very close proximity toeach other, as schematically illustrated in FIG. 3. here, the beam isattached similarly to the embodiment in FIG. 2, but the suspension isbasically a point attachment (at 9′ in FIG. 3) rather than by means ofsuspension member. Such a design will also yield the desired leverageeffect, but will be more difficult to manufacture.

[0025] Now an embodiment of a miniaturized pressure sensor according tothe present invention will be described with reference to FIG. 4, whichis perspective view with parts broken away for clarity.

[0026] It comprises a support body 10, e.g made of silicon althoughother materials are possible provided they can be micro-machined to thestructures required for achieving a properly functioning sensor. Adepression 12 is formed in the support body so as to provide a closedvacuum cavity when covered by a diaphragm 14. This diaphragm covers notonly the cavity, but the entire sensor surface, which has certainbeneficial effects, which will be discussed below. The diaphragm 14 isresponsive to external pressure by being deflected. A strain gauge 16 isattached at one end to said support body inside said cavity and beneathsaid diaphragm, and at another end suspended via a suspension element 18by said diaphragm, so as to be what is referred to for the purpose ofthis application as “free hanging”, as opposed to the prior art deviceshown in FIG. 1, where the strain gauge is attached on the diaphragm.

[0027] Thus, the sensor design consists of an integrated vacuum cavityas a pressure reference in which the strain-gauge is suspended. Thesensor is fabricated using only surface micro machining to allow smallchip sizes. Suitable dimensions of the fabricated pressure sensor arelisted in Table 1, although of course the dimensions can be varied asdesirable within the limits of the manufacturing technology.

[0028] In the shown embodiment, the pressure sensitive part is a squarepolysilicon diaphragm 14. A force transducing strain sensitive beam 16is attached to the support body inside the vacuum cavity at one end andto the diaphragm at the other, as shown in. The gap (d in FIG. 2)between the polysilicon beam and the diaphragm is 1.6 μm in thisembodiment. TABLE 1 Suitable dimensions of the fabricated pressuresensor. Diaphragm side length, a  100 μm Diaphragm thickness, t_(d)   2μm Cavity depth, h_(c)  2.5 μm Beam length, L   80 μm Beam width, w_(b)  30 μm Beam thickness, t_(b)  0.4 μm Gap between diaphragm and  1.6 μmbeam, h

[0029] The diaphragm layer covers not only the cavity but the entiresensor surface, as can be seen in FIG. 4. Thereby, the diaphragm createsa fluidic as well as an electrical barrier between the surroundingliquid and both the strain-gauge and the necessary electrical leadslocated in the embedded polysilicon layer.

[0030] In the shown embodiment, the force transducing beam has beenshaped in order for the current I to flow along one side of the beam andback along the other side, schematically shown by arrows. The current istaken up via contact pads 13, which are further connected to internalelectrical leads internal of the structure (not shown). The current pathhas been achieved in the illustrated embodiment, by providing “cut outs”20, 21 in the beam 16 so as to form two legs 22, 24 and a transversemember 26 connecting the legs 22, 24, and two further legs 27, 28 inwhich the suspension member 18 is attached. Thus the overall shape ofthe strain gauge or beam 16 is that of an “H”. In this way an insulatedcurrent path is created, and this enables an integration of thepiezoresistive strain-gauge in the polysilicon beam layer. The “H”-shapeis of course only exemplary, and any shape that exhibits the“leg-and-connector” structure would do, e.g. an “X”-shape. A “Y”-shapewhere the double leg end forms the two conductors and the single leg isattached to the suspension member would also be possible. Also a “U”shape is possible, if only one “cut out” is made, so as to form only twolegs in the beam member.

[0031] Thus, generally the legs of the strain gauge are attached to thesupport body, and the portion comprising the part where the legs areconnected to each other is attached to the diaphragm, i.e. at or beyondthe point where the legs are connected to each other.

[0032] The transverse connecting member is preferably located at adistance from the point of attachment in the support body thatcorresponds to at least 50% of the total length of the strain gauge.

[0033] The point where the strain gauge is suspended by the diaphragm islocated at a distance from the point of attachment in the support bodythat corresponds to at least 70%, preferably 80% of the width of thediaphragm in the direction of the strain gauge.

[0034] However, the point where the strain gauge is suspended by thediaphragm can also be located at a distance from the point of attachmentin the support body that corresponds to not more than 30%, preferablynot more than 20% of the width of the diaphragm in the direction of thestrain gauge.

[0035] In an alternative embodiment it is also possible to make astrain-gauge by diffusing an electrically conductive path into arectangularly shaped beam. However, in this case a current leakagebetween the legs of the strain-gauge will occur, reducing the efficiencyof the device.

[0036] Another option for making the current path on the beam is toprovide an insulating material in a pattern corresponding at least tothe current path (i.e. it can have a large extension than the pattern),and to provide a conducting material on top of said pattern, so as toform strain sensitive leads on the beam. The pattern can take basicallyany shape, e.g. any of the shapes recited above, or other more complexpatterns. Limiting factors on the conductor could be length, the longerthe conductor is the more heat would be generated, which could affectthe measurement.

[0037] The active strain sensing part of the H-shaped beam, is mainlythe first part, i.e. the loop consisting of legs 22, 24 and transverseconnecting member 26, since the measured resistance change in the secondpart, legs 27, 28, although it is as sensitive, is negligible compared(in absolute numbers) to the measured resistance change of the firstpart.

[0038] To achieve a high sensitivity and to take full advantage of theleverage arm (between the beam and the diaphragm at the attachmentpoint) a FEM-model was made to numerically calculate the sensorbehavior. The position of the diaphragm attachment point (the beamlength) was investigated for maximum pressure sensitivity. The highestsensitivity was reached with a beam length of 80 μm, and a squarediaphragm 100 μm×100 μm. For other geometries other optimal attachmentpoints can easily be found, by FEM calculations.

[0039] The expected output signal in a Wheatstone bridge configurationwith a supply voltage U_(bridge) for the leverage beam pressure sensoris$\frac{U_{out}}{U_{bridge}} = {\frac{\Delta \quad R}{4R} = {\frac{{ɛ_{x}G_{par}} + {ɛ_{y}G_{per}}}{4} \cong \frac{ɛ_{x}G_{par}}{4}}}$

[0040] were R is the resistance, of each of the four resistors, G_(par),G_(per) is the longitudinal and transverse gauge factors, respectively,and ε_(x) is the pressure induced strain numerically calculated to 0.3μ/mmHg. G_(par) is estimated to 22 and the longitudinal strain of theresistor is assumed to be dominating the contribution of the resistancechange.

[0041] The calculated pressure sensitivity (defined as the relativechange of output voltage versus applied pressure) for the free hangingforce transducing strain gauge pressure sensor is 2 μV/V/mmHg which isless than the measured sensitivity of 5 μV/V/mmHg. The difference isprobably due to the fact that the gauge factor is estimated incombination with the simplifications in the FEM-model which was buildfor optimization purposes.

[0042] Now a method of making a sensor according to the invention willbe described with reference to FIGS. 5-8. The fabrication process startswith defining the cavity 12 in the support body 10 by a KOH etching anda LOCOS oxidation, FIG. 5. A TEOS layer 50 is then deposited and definedto serve as etch channels. The strain-gauge structure is formed by alayer 52 of polysilicon and low stress silicon nitride 54, FIG. 6.Deposition and patterning of a new layer 56 of TEOS (FIG. 7) defines thespacing between the diaphragm and the strain-gauge structure as well asthe attachment point. The diaphragm 14 is made of a thin layer 58 of lowstress silicon nitride and a layer 60 of polysilicon. Electrical contactholes 62 and channels 64 for the etching solution (50% HF) are opened byRIE etching, FIG. 5c. The HF etches the silicon dioxide and TEOS,leaving the strain-gauge 16 suspended in a chamber. The chamber isvacuum-sealed with TEOS in a LPCVD process, FIG. 8. Finally, aluminum issputtered and patterned to define the metal conductors 64, FIG. 8.

[0043] The pressure sensitivity was measured by connecting thepiezoresistive strain gauge together with three external resistors in afull Wheatstone bridge configuration. The bridge was supplied with aconstant voltage of 8.3 V and the output signal measured using a HP34401A multimeter. The pressure sensitivity was measured by placing thesensor inside a pressure chamber and applying an overpressure from 0 to300 mmHg.

[0044] The results of the measurements are presented in FIG. 9. A sensorwith a diaphragm of only 100×100×2 μm and a H-shaped strain-gauge of80×40×0.4 μm has a high (for miniaturized sensors) measured pressuresensitivity of 5 μV/V/mmHg. The new fabricated free-hanging strain-gaugesensor has about 5 times higher sensitivity than a 1 μm thickrectangular leverage beam with a 0.4 μm thick piezoresistor 11 and about8 times higher than an earlier reported sensor with 100×100 μm diaphragmscaled to equal diaphragm thickness of 2 μm. The H-shape is preferredsince it realizes a piezoresistive strain-gauge and minimizes thestiffness.

[0045] The new design (X, Y, U or H shape) increases the pressuresensitivity compared to traditional and a rectangular shape forcetransducing beam design. The sensitivity for the ultra miniaturepressure sensor has been measured to 5 μV/V/mmHg. The sensor isfabricated using only surface micromachined processes and constitutes ofa 80 μm long H-shaped free hanging force transducing beam, locatedbeneath, a square 2 μm thick polysilicon diaphragm with a side length ofonly 100 μm. The new sensor enables a combination of high pressuresensitivity and miniature chip size as well as good environmentalisolation which makes it suitable for use in intravascular applications.

[0046] In another embodiment of the invention the leverage effect isachieved by suspending the force transducing beam in the diaphragm atboth ends, i.e. in two points on the diaphragm. Thereby a doubleleverage effect is attainable by suspending the beam on opposite sidesof the midpoint of the diaphragm, as shown in FIG. 10a and 10 b.

[0047] A deflection of the membrane will cause the suspension elementsto pull the beam in opposite direction, thereby further increasing thestrain, and thereby the sensitivity of the device. A drawback with thisembodiment is that electrical connections will have to be on thediaphragm, which could have a stiffening effect that would counteractthe increased sensitivity to some extent.

[0048] Piezoresistive detection inherently has a high temperaturedependence, which should be compensated for.

[0049] Prior art miniaturized pressure sensors with piezoresistivedetection commonly have two piezoresistors connected in a half bridge(or four piezoresistors connected in a full bridge) configuration fortemperature compensation. The resistors are either located on thepressure sensing diaphragm (of which one has a positive gauge factor andthe other have a negative) or one piezoresistor is placed on thediaphragm and the other on the substrate. In the latter configurationthe piezoresistor on the substrate is pressure-insensitive and only usedfor temperature compensation.

[0050] The differences in temperature induced strain could be caused bythe differences in the location of the piezoresistor and differences inthe material underneath the resistors. To obtain the same temperature inboth piezoresistors (and thereby be able to compensate for it) it isessential that the thermal environment for the two resistors is as equalas possible. The beam design according to the invention accomplishesthis by locating two similar beams within the same reference cavity andthus exposing them to almost identical thermal environment and alsolimiting the fabrication variations between them.

[0051] The piezoresistor on the beam is surrounded by vacuum(low-pressure), thus, any thermal transport occurs mainly through thebeam suspension points. A temperature compensation piezoresistor (notshown) located on the substrate 110, as illustrated in FIG. 1I (priorart), would therefore experience a different thermal environment thanthe pressure sensing strain gauge located on the leverage beam 112inside the vacuum cavity.

[0052] According to the present invention, instead the passivepiezoresistor (pressure insensitive) is located on a beam inside avacuum cavity too, preferably the same vacuum cavity as the activepiezoresistor (pressure sensitive), and preferably parallel to thepressuresensitive leverage beam, to create a thermal environment forboth, which is very similar.

[0053]FIG. 12 illustrates the new sensor design. The sensor 120comprises a pressure sensitive polysilicon diaphragm 121, and a forcetrancducing beam 122 (lower member in the figure), with a length ofpreferably 80% of the cavity side length, with a strain sensitivepiezoresistor is attached to the underside of the diaphragm at 123′. Theother end of the beam is attached to the cavity edge, indicated at 123″in the figure. Parallel and adjacent to the force-trancducing beam 122,another beam 124 (upper member in the figure) with a strain-gauge (notshown) is suspended in a manner that gives it a different pressuresensitivity. This property can be achieved e.g. by making it of adifferent length compared to the beam used for pressure measurement. Itcan either be shorter or longer, preferably 20% or 100% of the cavityside length. Alternatively this second beam can be attached at both endsto the cavity edge, at 125, 126 respectively. It is also possible tosuspend both beams in the diaphragm at different locations to achievethe desired difference in sensitivity. Both the beams and thestrain-gauges are fully enclosed inside the cavity, which functions as apressure reference.

[0054] The new design with the dual beam accomplishes an almostidentical thermal conductance between the piezoresistors and thesubstrate. Under the assumption that the heat transfer from the pressuresensing piezoresistor, through the beam, the attachment-point and thediaphragm to the fluid is negligible the thermal differences of thepiezoresistors are also independent of the flow. Since bothpiezoresistors in the dual beam design are located on similar beams thenew design also makes the differences in the temperature inducedsensitivity smaller than for the traditional design.

[0055]FIG. 13 shows the low relative temperature dependency mismatch forthe piezoresistors in the new design (bottom curve) compared to thetraditional design (top curve).

[0056] The invention having been thus described is subject to variousalterations and modifications and the skilled man would arrive at suchalterations without inventive skills. E.g. other geometries of the beamare conceivable within the scope of the claimed invention.

1. A miniaturized pressure sensor, comprising a support body (10) havingan upper surface and a depression (12) formed in said surface; adiaphragm (14), covering said depression (12) so as to form a closedcavity, said diaphragm being responsive to external pressure by beingdeflected; a strain gauge (16) disposed inside said cavity and beneathsaid diaphragm (14), cooperatively coupled to said diaphragm so as togenerate an output signal in response to the deflection of saiddiaphragm, said strain gauge in at least one of its ends being suspended(18) by said diaphragm.
 2. The sensor as claimed in claim 1, whereinsaid cavity is evacuated.
 3. The sensor as claimed in claim 1 or 2,wherein the diaphragm material covers the entire upper surface of thesupport body.
 4. The sensor as claimed in claim 1, 2 or 3 wherein thestrain gauge is formed from a piezo resistive material.
 5. The sensor asclaimed in any preceding claim, wherein the strain gauge comprises twolegs (22, 24), connected to each other, said legs together forming acurrent conducting path (I).
 6. The sensor as claimed in claim 5,wherein said legs are connected by a transverse member (26).
 7. Thesensor as claimed in any of claims 1-5, wherein the strain gauge has thegeneral shape of an “H” or an “X” or a “Y” a “V” or a “U”.
 8. The sensoras claimed in any of claims 5-7, wherein the legs of the strain gaugeare attached to the support body, and wherein the other end, i.e. at orbeyond the point where the legs are connected to each other is attachedto the diaphragm.
 9. The sensor as claimed in claim 6, wherein thetransverse connecting member is located at a distance from the point ofattachment on the support body that corresponds to at least 50% of thetotal length of the strain gauge.
 10. The sensor as claimed in anypreceding claim, wherein the point where the strain gauge is suspendedby the diaphragm is located at a distance from the point of attachmentin the support body that corresponds to at least 70%, preferably 80% ofthe width of the diaphragm in the direction of the strain gauge.
 11. Thesensor as claimed in any of claims 1-9, wherein the point where thestrain gauge is suspended by the diaphragm is located at a distance fromthe point of attachment in the support body that corresponds to not morethan 30%, preferably not more than 20% of the width of the diaphragm inthe direction of the strain gauge.
 12. The sensor as claimed in claim 1,wherein the strain gauge comprises a support beam, and a layer of apiezoresistive material disposed on said beam in a pattern forming acurrent path.
 13. The sensor as claimed in claim 12, wherein saidpattern has a general shape of a “U”.
 14. The sensor as claimed in anypreceding claim, wherein there is provided a second strain gauge (124)in said cavity, the second strain gauge having a different pressuresensitivity than the other strain gauge (122).
 15. The sensor as claimedin claim 14, wherein said second strain gauge (124) is arranged paralleland adjacent to the other strain gauge (122).
 16. The sensor as claimedin claim 14 or 15, wherein said second strain gauge has a length thatdiffers from the length of the other strain gauge.
 17. The sensor asclaimed in claim 14, 15 or 16, wherein said second strain gauge isattached at both ends in the support body.
 18. A miniaturized pressuresensor, comprising a support body having an upper surface and adepression formed in said surface; a diaphragm, covering said depressionso as to form a closed cavity, said diaphragm being responsive toexternal pressure by being deflected; a strain gauge attached at one endto said support body inside said cavity and beneath said diaphragm, andat another end suspended by said diaphragm.
 19. A miniaturized pressuresensor, comprising a support body with an evacuated cavity formedtherein, a diaphragm forming a sealing cover for said cavity, and astrain gauge disposed inside said cavity and coupled to the support bodyand said diaphragm such that a deflection of said membrane caused by apressure exerted thereon causes a detectable change in strain in saidstrain gauge.
 20. A miniaturized pressure sensor, comprising a diaphragmresponsive to pressure by being deflected, and a free-hanging straingauge responsive to strain by yielding an electrical output signal, saidstrain gauge being suspended at one end by said diaphragm, such that adeflection of said diaphragm causes a detectable change in strain insaid strain gauge.
 21. The miniaturized pressure sensor as claimed inclaim 20, wherein said diaphragm sealingly covers an evacuated cavity,and wherein said free-hanging strain gauge is located inside saidcavity, beneath said diaphragm.
 22. A miniaturized pressure sensor,comprising a support body with an evacuated cavity formed therein, adiaphragm forming a sealing cover for said cavity, and a forcetransducing beam provided inside said cavity and beneath said diaphragm,and suspended by said diaphragm in at least one end of said beam by asuspension element having a finite length.
 23. The miniaturized pressuresensor as claimed in claim 22, wherein the suspension element isarranged to cause a leverage, when the diaphragm is deflected due to apressure exerted thereon, so as to move the beam correspondingly.
 24. Aminiaturized pressure sensor, as claimed in any of claims 1-7, or claims12-23 as dependent on claims 1-7, wherein there are provided suspensionelements having a finite length by which said beam or beams is/aresuspended by said diaphragm in two points located on opposite sides of amidpoint of said diaphragm.